Command Line Flashcards
What if the UNIX Shell?
Both a command-line interface (CLI) and a scripting language
What is the function of the UNIX shell
A better GUI alternative that allows repetible tasks to be done automatically and fast. Tasks that takes the GUI hours will take the shell seconds.
What is the shell?
A program where users can type commands ranging from complicated climate modeling software to simple file creations
What is the most popular UNIX Shell
Bash (Bourne Again Shell)
What are scripts? Purpose?
Sequences of commands written using the grammar of a shell to improve the reproducibility of workflows.
Examples of the shell abilities?
Can be used to interact with remote machine and super computers. It can also be used to tackle scientific questions and computational challenges.
What is $? What to do and not?
This is the shell’s prompt typically. Do not type the prompt, only the commands after it. Press enter to execute it.
What is the [ls] command and what does it mean?
[ls] - short for listing. This lists the contents of the current directory
What happens when shells can’t find ur command?
$ (command): command not found
What are the shells main disadvantages?
The shell’s main disadvantages are its primarily textual nature and how cryptic its command and operations can be.
What is the file system?
The part of the OS responsible for managing files and directories.
Current working directory
Directories are like places. At any time we r using the shell we are exactly in one this.
Why knowing ur current working directory is important?
Command mostly read and write files in the current working directory
What is the [pwd] command?
Shows you where you are
What is the [cd] command?
Navigates you to ur home directory
[/]
Root directory that hold everything else it’s the leading slash in ur home directory
What other directories are inside the root directory?
[bin], [data], [Users], and [tmp] etc.
Function of the [bin]
This is where some built-in programs are stored.
Functions of [data]
(For miscellaneous data files)
Functions of the [Users] directories
It’s where the user’s personal directories are located.
Functions of the [tmp] directory ?
For temporary files that don’t need to be stored longterm
The seconding meaning of the slash?
When it appears inside a path, it’s just a separator
Where are user Imhotep files stored in?
They are stored in all user’s directory
[ -F] option
Tells [ls] to classify the output by adding a marker to file and directory names to indicate what they are.
What does this [/] indicate as a marker by a file?
A trailing [/] indicates that this is a directory.
What does this [@] indicate as a marker by a file?
Indicates a link
What does this [*] indicate as a marker by a file?
Indicates an executable
What are plain old files?
Any names in the output without a classification
What are sub-directories?
Directories under the main directories under the root
How to clean a cluttered terminal?
[clear] command
How to access previous commands?
Up and down to retrieve past lines
How to find out what options a command have and what it does?
[ —help]
What is the error for unsupported option?
ls: invalid option — ‘j’
Try ‘ls —help’ for more information
Another way to find out what options a command have and what it does?
[man ls]
What does [man ls] do differently?
Turns ur terminal into a page with a description of the command and its option.
How to search for something in the [man] page?
use [/] followed by the character or word you are searching for.
How to quit the [man] page?
Press [Q]
[ -l] option function
Long list format of the file and directory names
With details like file size and last time or modification
[ -h] option function
Makes the file size/(numbers) human readable. 5.3K instead of 5369
In what order does [ls] list the contents?
Alphabetically
How does the [ -t] option for the ls command arrange the directory?
By the time it was last edited
How does the [ -r] option for the ls command arrange the directory?
Reverse order
How to get a listing of something other then our CWD? Name the parts.
[ls -F desktop]
[ls] - lists stuff
[ -F] - option
[ Desktop] - argument
What does argument do as used in [ls -F Desktop]?
The argument [Desktop] sets a parameter. Tells [ls] that we want a listing of something other than our current working directory.
How to look at the contents of a sub-directory of non-cwd in ur cwd?
[ls -F argument/name of directory
Describe the cd progression?
To change the shells cwd u use [cd]
You can only go down into the directory tree within the cwd or to the home directory
How to go up a directory tree?
[cd . . ]
[cd . .] meaning?
Special directory meaning the directory containing this one. The parent of the cwd.
How to show the special [ . .] when u run [ls]?
add [ a] option to [ls -F]
What does the special directory [.] mean?
CWD
Purpose of the hidden [ .bash_profile]
For file configuration settings. Files and directories with the [.] prefix before them are to prevent the config files from cluttering the terminal.
How to go down 3 steps down the directory levels with one command line?
[cd directory name/sub/sub]
This can go down for as long as there is a sub directory to access
How does the shell interprets the tilde ([~]) character?
Means the current user Cwd
=/Users/user’s name/data
How does the shell interprets the tilde ([-]) character?
Translate into the previous directory u were in
[$ ls -F / ]
[$ ] - Prompt
[ls] - Command
[ -F] - Option
[ /] - Argument
What is the functions of arguments?
Tell the command what to operate on (e.g files and directories)
Another name for arguments
Parameters
How many option and argument in a command?
Multiple
Do commands require always parameters?
No
Switch/flags
Options and mostly those that take no argument
What is the command sensitive about?
Case and Space sensitive
What does an absolute file specify?
A location from the root of the file system
What does a relative path specify?
Location starting from the current location
What is the command to make a directory?
[mkdir]
Why is [mkdir] a relative path?
Does not have a leading slash and def
[ -p] option, function
Allows [mkdir] to create a directory with nested subdirectories in a single operation.
[ -R] option, function
This option to the ls command will list all the nested sub directories within a directory
Why shouldn’t u use spaces when making directories and alternative?
Because they are used to make separate arguments use (-), (_) , and (.)
Should u begin a name with (-)?
No
How to refer to directories with special characters, spaces, etc
(“”)
What [nano] really means?
(Text) it can only work with plain character data. One of the least complex text editors
Explain the control key
Ctrl, ^, C
What does the [touch] command do and why use it?
The touch command generates a new file in your cwd. It’s can be used for programs that need help generating empty files.
What does the [rm] command do?
Removes a file
What are filename extension?
Second part of a file name that helps people and their programs tell the different kind of files apart
(.txt)
Signals a plain text file
(.pdf)
Indicates a PDF document
(.cfg)
Configuration file full of parameters for some programs or other.
(.png)
PNG image
What does the [mv] command do?
It moves files and directories but it can also rewrite them.
How to move a file or directory?
The first argument tells [mv] what we’re moving and the second where to go.
How to change file or directory name?
The first argument after [mv] will be the path or file name to the ur the file u want to change in the cwd and the second will be the path or file name that will override it
[ -i] or [ —interactive] option, function and purpose
[mv] will silently overwrite any existing file with the same name. This option makes it ask for permission
What does the [cp] command do?
It copies files
What does the recursive option [ -r] do?
It copy a directory and all its contents. You can use to backup a directory.
Dangers in using the [rm] command?
Deleting is forever use the [ -i] to be safe tho
How to remove directories and all their options?
[rm] itself can’t remove directores, adding the recursive [ -r] option will do that.
How to copy multiple files at once?
Put the files u want to copy with space in between them, the last argument has to be a directory name
What does [*] wildcard do?
Matches zero or more character in a filename
How to use [*]?
Use with ls command. [*] to replace missing character or characters
What does the [?] do?
Matches only one character.
How to use [?] wildcard?
Use [ls] then [?] To replace a missing word
[?] vs [*]
[] Gives u the file(s) that contain ur looking for and any other that matches it . Kind of a substitute for all the words that might be after the []
[?] Gives u the file(s) that contain the words behind the [?]. Kind of a substitute for one word that might be after the [?]
What does the (.pdb) extension indicates? Function?
That files are in Protein Data Bank format. A simple text format that the type and position of each atom in the molecule
What does the [wc] command stand for and what does it do?
Word count, it counts the number of line, words, and character (from left to right, in that order)
[wc -l], function
The output shows only the number of lines per file
[wc -m], function
Shows the number of characters
[wc -w], function
Show’s the number of words in the files
What happens when given a command to process a file but given no file name?
Sits there and waits for u to gives it data and appears to do nothing. Escape this state with ctrl + c
What does the [>] symbol tells the shell to do?
It tells the shell to redirect the command’s output to a file instead of printing it out.
When does the shell override a file?
When u tell it to create a file with a name that already exist
What does the [cat] command do? Where does it name come from?
It gets ur name from concaténate. It prints out contents of files one after the other
A more useful version of [cat]?
[less]
[less] function?
Displays a screenful of the file, and then stops.
How to operate the [less] command?
Go forward - spacebar
Back one - b
Q - quit
What does the [sort] command do?
Sorts alphanumerically
What does the [sort -n] do?
Sorts in a numerical order
What does the [head] command do?
To get the first few lines in the file.
Needs the [ -n #] option tho
# - sub for a number.
What does the command [head] do with [ -n 1] option? Another example?
Tells head we only want the first line. [ -n 20] would be the first 20 line
Why not redirect to the same file?
Might give incorrect result or delete the contents of the file
What does the [echo] command do?
The echo command prints text
How to write to files using the [echo] command?
Use the greater than sign [echo] (what ur writing) [>] or [»] and then (name of the file ur writing to.
When using echo to write to files what’s the difference between [>] and [»]?
[>] rewrites the entire file
[»] appends to the file
What does the [tail] command do?
- sub for a number.
Just like the [head] command the [tail] prints lines from the end of a file. Needs the [ -n #] option tho
What is a pipe? Purpose?
This is the vertical bar [|]. Put between two commands. It tells the shell we want to use the output of the command on the left as the input of the command on the right
What is a filter?
A filter is a program like [wc] or [sort] that transforms a stream of input into a stream of output.
What is the purpose of the [cut] command? What does it expect?
It’s to remove certain sections of each line in the file and [cut] expects the lines to be separated into columns by a (Tab) character.
What is a delimiter?
A character used to separate lines into columns.
What does the [ -d] do?
To specify what the delimiter is
Purpose of the [ -f] option?
To specify which row you wish to remove
How to isolate sections of a file with lines?
[cut -d (specify what separates ur columns) -f (which column u want out) (name of the file)]
What does the [uniq] command do?
Filters out adjacent matching lines in a file
What does the [uniq]’s command [ -c] option do?
Gives count to the number of times a line occurs in its input
Write the general loop form?
[For (thing) in (list_of_things)
do
(operation_using) $thing
done]
Indentation not needed but helps readability.
Prompts in loop, change purpose?
The shell prompt changes from [$] to [>] and back again as we type the loop. The [>] prompt is to remind us we haven’t finish typing a complete command.
What does the [;] colon doing?
Used to separate two commands written on the same line.
What does the [$] in the general loop for do?
Calls for the variable’s value by putting [$] in front of it. Tells the shell interpreter to treat the variable as a variable name and substitute it’s value in its place, rather than treat it as text or an external command.
How to delimit variables?
Use the curly brackets. {}
[$filename] = [${filename}]
[${filename}] not = [${file}name]
Rules for variable?
Give them meaningful name
How to move to the beginning and end of the shell
Begin - [ Ctrl + A ]
End - [Crtrl + E]
What is the [history] command?
Get a list of the last few hundred commands that have been executed
How to repeat a command after the [history] command?
[!(#)]
What does [!$] do?
Retrieves the last word of the last command.
What does the Ctrl + r shortcut do?
Entera a history search mode ‘reverse-i-search’ and finds the most recent command in your history that matches the text you enter next.
What is a dry run?
A cushion for mistakes. Use the [echo] command to show what a loop would do. Put the commands in double quotations.
Why give files consistent names?
Easy to match with wildcard pattern to make it easy to select them for looping.
What is a shell script?
A bunch of commands saved in a file is usually called a shell script. Small programs.
What does the [bash] command do?
Tells our file to execute the program
Why must one use plain text editor?
Shell tools only expect files to contain nothing but letters, number and special characters
What does [“$1”] means? Why put double quotes around it?
The first filename (or other argument) on the command line. In case it has spaces or confusing special characters add double quotes
What does [$2] and [$3] mean?
Second and third command line argument respectively
What is a comment? Purpose? Tip
A comment starts with a [#] character and runs to the end of the line. The computer ignores them but people use them to understand your code. Each time u modify ur script check if the comment is still accurate.
What does the [$@] command mean? Purpose?
All the command-line argument to the shell scripts. Equivalent to [“$1” “$2” …]
Add quotations. Use to manipulate all sort of files including those in other directories.
What happens when u use single quotations?
The shell does not expand the command to others and only does the command in the quote
What does the [ -x] option do?
It reruns the script. Runs in debug mode.
What does the [grep] command do?
Finds and prints lines in files that match a pattern.
How to use a [grep] command?
[grep (pattern) filename]
What does the [ -w] option does with the [grep] command?
It restricts matches to line containing the word on its own. Word boundaries. This also applies to multiple words and spaces. Quotes is useful for multiple words
What does the [ -n] option do for the [grep] command?
Gives u the line number for the lines that match.
What does the [ -i] option do for the [grep] command?
Makes the search case sensitive
What does the [ -v] option do for the [grep] command?
Inverts the search. Looks for line that does not contain the word(s)
What does the [ -r] (recursive) option do for the [grep] command?
Can search for a pattern recursively through a set files in subdirectories.
Where does [grep] real power come from?
Regular expressions. The ‘re’ in [grep].
Function of regular expressions
Both complex and powerful
When using [grep]’s regular expression whats the purpose of [ -E]?
To prevent the shell from trying to interpret the command. Not enough on its own
What does [“^.o”] do? Why?
If [*] is used without quotes the shell would try to expand it.
^ - is the pattern anchors the match to the start of the line
. - Matches a single character
o - matches an actual o
What does the [find] command do?
Finds command files themselves
What does the [ -type d] option do for the [find] command?
It lists the directories including the cwd with no particular order
What does the [ -type f] option do for the [find] command?
List the files with no particular order
What does the [ -name] option do for the [find] command?
Can find all the files that contain the letters given and if there are missing letters use [*] as subs
What does the [$()] command do
The shell executes this first then replace it with its output
